Preprint typeset using LATEX style emulateapj v. 5/2/11 EXCLUSION OF COSMIC RAYS IN PROTOPLANETARY DISKS. II. CHEMICAL GRADIENTS AND OBSERVATIONAL SIGNATURES

The chemical properties of protoplanetary disks are especially sensitive to their ionization envi-ronment. Sources of molecular gas ionization include cosmic rays, stellar X-rays and short-lived radionuclides, each of which varies with location in the disk. This behavior leads to a significant amount of chemical structure, especially in molecular ion abundances, which is imprinted in their submillimeter rotational line emission. Using an observationally motivated disk model, we make pre-dictions for the dependence of chemical abundances on the assumed properties of the ionizing field. We calculate the emergent line intensity for abundant molecular ions and simulate sensitive observa-tions with the Atacama Large Millimeter/Sub-millimeter Array (ALMA) for a disk at D = 100 pc. The models readily distinguish between high ionization rates (ζ & 10−17 s−1 per H2) and below, but it becomes difficult to distinguish between low ionization models when ζ. 10−19 s−1. We find that H2D + emission is not detectable for sub-interstellar CR rates with ALMA (6h integration), and that N2D + emission may be a more sensitive tracer of midplane ionization. HCO+ traces X-rays and high CR rates (ζCR & 10−17 s−1), and provides a handle on the warm molecular ionization properties where CO is present in the gas. Furthermore, species like HCO+, which emits from a wide radial region and samples a large gradient in temperature, can exhibit ring-like emission as a consequence of low-lying rotational level de-excitation near the star. This finding highlights a scenario where rings are not necessarily structural or chemical in nature, but simply a result of the underlying line excitation properties. Subject headings: astrochemistry — circumstellar matter — protoplanetary disks — radiative transfer — stars: pre-main sequence 1